Forty-three cats (experiments 1 and 2) were vaccinated (2 doses, 27 and 30 days between doses) with the F-9 strain of feline calicivirus by the intramuscular route. There was no untoward response in any of the cats to the administration of the vaccinal virus nor was there spread of the virus from 20 vaccinated cats to nonvaccinated cats held in contact during the next 6 months (experiment 2). The vaccinated cats developed serum-neutralizing antibodies that were increased further after the 2nd vaccination. The level of serum-neutralizing antibodies was related to the quantity of vaccinal virus administered. Twenty-three cats vaccinated with the F-9 strain were protected to a significant degree when challenge exposed to virulent calicivirus strain FPV-255 (experiment 1).